Texture size and creating

[m1lkm8n 410]

So I've got a terrain with 1024 x 8 grid size. I haven't seen much on the texture size explanations so what is an appropriate texture size for this map?

I notice when binarize everything and I'm in game I can see the texture repeating. It's very noticeable.

Also if anyone can point me in the direction of a good tutorial on creating your own textures. I've tried betons tutorial but the newer version of gimp doesn't match up and I'm stuck on the 3d preview part as I can't find that listed under filter.